1. Field of the Invention
This invention relates to a chemical gas generating composition in solid, pelletized form, which is capable, upon ignition, of rapidly producing large volumes of non-toxic gases. The gas generating composition of the invention is particularly adapted for inflating safety crash bags in passive restraint systems for passenger vehicles.
2. Description of the Prior Art
The use of protective gas inflated bags to cushion vehicle occupants in a crash situation is now widely known and well documented. In the early systems of this type, compressed, stored gas is used to inflate a crash bag which is situated between the occupant and the windshield, steering wheel, and dashboard of the vehicle. Upon rapid deceleration of the vehicle, gas is released through a quick acting valve to inflate the crash bag. Stored, pressurized gas systems have now been largely superceded by systems which utilize gases generated by the ignition of a chemical gas generating pyrotechnics substance. In such a system, an ignition means, such as an electrically activated squib associated with a suitable sensing means, is used to ignite the gas generating composition.
Because of the recent strict requirements for non-toxic inflating gases, most, if not all, of the gas generating compositions now in use are based on inorganic azides. Early azide compositions did not address the toxicity problem of the gases generated since it was not envisioned at that time that the gases generated should be breathable. Gas generating compositions which will provide non-toxic gases or those which will burn at a high burn rate are easy to prepare, but it is difficult to meet the requirements of rapid burn rate and non-toxic gas generation in a single gas generating composition.
Metal oxides as coreactants for an inorganic metal azide have been used to produce non-toxic nitrogen gas for inflating crash bags. In the reaction with the inorganic azide, the metal oxide provides the oxygen for conversion of the inorganic metal in the azide to an oxide. The metal oxide additionally reacts to form a sintered, coherent combustion residue which is easy to filter. However, the use of metal oxides as coreactants for inorganic azides present several problems, one of which is that such mixtures are generally slow burning, difficult to form into pellets, and often cause rapid wear of punches, dies, and other parts of automatic pelletizing machines.
The formation of a gas generating particulate composition into pellets has generally been found to provide uniformly burning propellants, which are necessary for proper inflation of the restraint device. When metal oxides are used as coreactants for inorganic azides in gas generating compositions, the pellets of the composition must be made very small or very thin to provide sufficient surface area to obtain a reasonable rate of combustion. Such pellets are difficult to produce and are subject to more breakage than larger pellets. While the addition to the gas generating composition of more effective oxidizer compounds, such as the alkali metal perchlorates or alkali metal nitrates, can be used to increase the combustion rate of metal oxide/inorganic azide gas generating compositions, these oxidizer compounds increase the combustion temperature so that the compositions, upon combustion, provide residues which cause filtration problems.
Many quick-burning gas generating compositions have been proposed in the prior art for crash bag inflation purposes. It is desirable to develop a gas generating composition combining the features of short induction period, a rapid burn rate, a high bulk density and the production of only non-toxic gases. In addition, good filterability of the reaction products is desirable so that the hot, solid residue cinders of the reaction of the pyrotechnic substance can be easily removed from the gas stream.
In U.S. Pat. No. 4,376,002 to Utracki, improved burn rate pyrotechnic compositions, suitable for inflating a gas bag, are disclosed in which a synergistic primary oxidant component, such as iron oxide, is utilized with an alkali metal or alkaline earth metal azide in combination with a residue control agent consisting of at least one of the oxides of titanium, aluminum, and zinc.
In U.S. Pat. No. 4,547,235, rapid burn rate pyrotechnic compositions suitable for inflation of a crash bag are disclosed utilizing sodium azide as the nitrogen source and wherein silicon dioxide and potassium nitrate are utilized in combination therewith to provide a gas generating composition having a rapid burn rate.
In U.S. Pat. No. 3,996,079 to DiValentin, pyrotechnic gas generating, granular compositions, suitable for inflating an air bag of an automobile passive restraint system, are disclosed in which an alkali metal or alkaline earth metal azide is utilized in combination with nickel oxide or iron oxide and a minor amount (0.5% to 3.0%) of clay. The clay is disclosed as needed to improve the extrusion characteristics, the burning profile, mechanical strength and packing density of the granular compositions. The gas generating compositions react at a relatively low temperature and the solid products of the reaction form a sinter which is readily retained by a filter. There is no indication that the addition of clay provides any effect upon the burning rate of the pyrotechnic composition.
In 3 U.S. Pat. No. 4,696,705 to Hamilton, a pyrotechnic gas generating composition is disclosed in which 0 to 5 percent by weight of bentonite clay is utilized. The pyrotechnic composition is disclosed as a coated grain having a coating weight of 1 to 4 percent of the total weight of the grain prior to coating. There is no indication that the use of bentonite provides any advantages in the composition.